Field of the Invention
The invention relates to bis-o-nitrophenol derivatives, to polybenzoxazoles that can be obtained from them, to the use of these materials as a dielectric or as a buffer layer, as well as to a microelectronic device that includes such a material.
In microelectronics, high-temperature stable polyimides and polybenzoxazoles are used, for example, as dielectrics and/or buffer layers. Precursors of polybenzoxazoles, so-called poly-o-hydroxyamides, can also be rendered photoreactive if suitable photoactive components are added to the formulation of these dielectrics. By heat treatment (heating) at temperatures in excess of 250° C., a poly-o-hydroxyamide can be converted into a polybenzoxazole.
The mechanism taking place during the cyclization of poly-o-hydroxyamides to form polybenzoxazoles is schematically represented as follows:

When heated, the o-hydroxyamide cyclizes to form the oxazole by eliminating water.
Besides thermal and mechanical stability, the dielectric constant of these materials is an important criterion, especially for use as a dielectric. It must be as low as possible, so that the electrical insulation effect is good, for example between interconnects or interconnect planes in microchips, and the electrical performance of the microelectronic device is enhanced. Polybenzoxazoles and/or poly-o-hydroxyamides are preferred over polyimides because they regularly have a lower dielectric constant than polyimides.
Compared with polyimides, photostructurable poly-o-hydroxyamides and/or polybenzoxazoles also have the advantage that they are positively structurable. This leads to a lower occurrence of defects, because only a small part of the layer needs to be illuminated in most cases. In addition, they are aqueous-alkalinically developed, whereas polyimides are usually developed by using organic solvents. For the production and/or disposal of the materials, it is always more favorable for a component not to consume any organic solvents, which regularly need to be disposed of separately.
In order to achieve a high resolution, that is,to be able to represent even small structures, illumination instruments that operate at short wavelengths, for example at 248 nm or less, are used. However, most of the buffer layers (coatings) that are used absorb so strongly, even at this wavelength, that sufficient illumination of the added photoactive component as far as the lower regions of the layer is scarcely still possible. This problem can be solved by increasing the transparency of the layer, in particular the transparency of the base polymer of this layer. The transparency of the poly-o-hydroxyamides, and/or of the polybenzoxazoles obtained from them, is therefore of particular importance for their suitability in microelectronics.
European Patent Application No. EP 0 317 942, which corresponds to U.S. Pat. Nos. 4,939,215 and 4,845,183, discloses bis-o-nitrophenols that are used as monomer units for the preparation of polymeric benzoxazoles. However, the polymers obtained in this way show very strong absorptions in the range of 248 nm and less, so that they are suitable only for illumination instruments with a longer wavelength. Furthermore, the polymers disclosed therein exhibit such high dielectric constants that they cannot be used as dielectrics in microelectronics.